1. Field of the Invention
The present relates generally to circuitry for generating a signal which is active when a relay is being released, for eliminating noise caused by the operation of relay contacts and thereby reducing or eliminating the susceptibility of electronic circuitry to such noise. Such a generated signal may be used in powering down the signal source to the relay contacts, e.g. dry relay switching, or for masking data collection while a relay is releasing. Specific applications are telephone line circuits, data acquisition and transmission systems and electronic circuitry generally which can be adversely affected by the interference caused by relay releasing noise.
2. Description of the Prior Art
It is known in the prior art that when relay contacts are used to break current flow or to connect unequal voltage potentials, arcing and showering may occur. Some types of relay contacts bounce when being connected, which prolongs and exacerbates the aforementioned condition. As a result, high frequency disturbances permeate through nearby circuitry via metallic connections, capacitive coupling and electromagnetic radiation. These disturbances can cause noise and data errors in such nearby electronic circuitry.
Telephone subscriber line circuits, for example, often use relays for disconnecting the battery feed circuit and for switching ringing voltage onto the subscriber line. When ringing is removed from the line, one set of relay contacts must break the current path of the ringing voltage while another must remake the current path to the battery feed circuit. Both of these actions can cause spurious tripping of sensors and noise impulses in nearby circuits.
Capacitors are commonly employed around relay contacts to suppress electrical disturbances. The size and effectiveness of these capacitors is often limited due to the need for high frequency as well as high voltage isolation.
U.S. Pat. No. 3,558,910 describes a known relay circuit employing a triac to prevent arcing. U.S. Pat. No. 4,146,752 describes a bounce free contact operation using a transistor which is rendered increasingly current conductive in accordance with a charging process setting. U.S. Pat. No. 4,300,024 describes a measurement circuit which overcomes the effects of relay contact bounce. U.S. Pat. No. 4,438,472 describes a dc arc suppression circuit which prevents contact bounce on closure of contacts by providing a diode in parallel with a portion of the circuit. U.S. Pat. No. 3,560,796 describes a relay control circuit which ensures that relay contact bounce occurs during a portion of the applied ac current source so as to eliminate contact erosion.